Polymers are a mainstay of modern society, for example, widely used in fabricating textiles, upholstery, construction materials, various air, land or sea vehicles, and microelectronic devices and appliances. The inherent flammability of many polymers poses a significant threat, especially in enclosed or isolated spaces. Therefore, as synthetic polymers are used extensively in society as plastics, rubbers, and textiles, polymer flammability has been recognized as a safety hazard and remains an important challenge in polymer research.
Flame retardancy of polymers is often achieved by blending polymers with flame retardant additives, such as halocarbons, including polybrominated diphenyl ether (PBDE), phosphorus, organophosphates, and metal oxides. While small molecule flame retardant additives provide a convenient means for reducing flammability of polymers, these additives can compromise safety from environmental and health perspectives. Conventional flame retardants are small molecule additives that often leach out of the polymer during their use leading to a variety of serious health and environmental problems associated with toxicity and bioaccumulation. These concerns have led to an emphasis on non-halogenated flame retardants in recent years. However, non-halogenated flame retardant additives, such as alumina trihydrate, compromise the physical and mechanical properties of polymers when loaded at high levels.
An ideal low-flammable polymer would be halogen-free and possess high thermal stability, low heat of combustion, and a low combustion heat release rate (HRR), with minimal release of toxic fumes. Intrinsically fire-resistant polymers that undergo significant carbonization upon heating are highly desirable, as carbonaceous char formation effectively averts combustion by producing an insulating layer on the polymer surface. Such char formation can also be realized from composite materials in which an additive ultimately provides the desired char.
Deoxybenzoin moieties have demonstrated utility as flame retardant materials, for example when incorporated in polyarylates, e.g., polyarylates based on 4,4′-bishydroxydeoxybenzoin (BHDB), as a bisphenolic monomer. Such polymers exhibited low combustion heat release rate and total heat of combustion, which is believed to arise from the thermally-induced conversion of BHDB to diphenylacetylene moieties that char by aromatization. See, K. A. Ellzey, T. Ranganathan, J. Zilberman, E. B. Coughlin, R. J. Farris, T. Emrick, Macromolecules 2006, 39, 3553. Pyrolysis combustion flow calorimetry (PCFC), an oxygen consumption technique for measuring heat release capacity (HRC), revealed exceptionally low HRC values for the BHDB-polyarylates (<100 J/g-K). See, R. N. Walters, M. Smith, and M. R. Nyden, International SAMPE Symposium and Exhibition 2005, 50, 1118.
It would be advantageous to identify new synthetic methodologies for integrating deoxybenzoin moieties into polymeric materials. Accordingly, there remains a continuing need for synthetic strategies toward structurally and functionally diverse flame-retardant materials, including high molecular weight polymers and crosslinked materials.